Nano-Optical Imaging Spectroscopy:

Recent Advances in Near-field Scanning
Optical Microscopy
Toshiharu Saiki* and Yoshihito Narita**

Abstract

1. Introduction

Current progress in the instrumentationand measurements of near-field scanning optical microscopy (NSOM) is described. The mostcritical element in NSOM is an aperture probe,
which is a tapered and metal-coated opticalfiber. The design and fabrication of the probe
are examined with regard to aperture quality
and the efficiency of light propagation. The
recent dramatic improvements in spatial resolution and optical throughput are illustrated
by selected applications, including ultrafast
coherent spectroscopy, Raman spectroscopy,
polarization microscopy, and the emission imaging of semiconductor nanostructures andsingle molecules. These examples demonstratethat NSOM has developed into a powerful tool
for study on a new frontier of science.

As an eye on the nanoscale world, optical
microscopy with high spatial resolving power
is indispensable in a wide range of science andtechnology. In material science, for example,
local optical properties that depend on the microscopic complexity and heterogeneity ofmaterials need to be characterized on the nanometer scale. And the nanoenvironment ofbiosystems can be identified by monitoring thefluorescent behavior of an individual molecule.
While the conventional optical microscope provides excellent spectral and temporal selectivity, its spatial resolution is limited by diffraction
to roughly half the wavelength of the illuminating light. Today, however, we need tools thatcan locate, distinguish, and characterize features whose size and separation are small com-

(a)

(b)

Sharpened fiber

Sharpened
metal tip

Metal film

Opticalnear-field

Aperture

Opticalnear-field: Illumination light(c): Signal light

Fiber probe

2. General description of
NSOM

Laser

Detector
SampleFeedback
control unit

Spectrometer

pared to the wavelength of light. Electron microscopy, scanning tunneling microscopy (STM),
and atomic force microscopy (AFM) provide
atomic-scale resolution and can be used tomanipulate atoms and nanostructures but are
not particularly suitable for the observation ofspectral and dynamic properties.
Resolution beyond the diffraction limit isusually obtained by using near-field scanning
optical microscopy (NSOM), in which a small
aperture (a scatterer) providing illumination isscanned close to the object under observation.1,2) The progress in probe fabrication techniques has made routine procedures of optical imaging and spectroscopy with a resolution of 100 nm. It is also possible, with a highquality probe, to reach a resolution of approximately 10 nm. The highly resolved topographic
mapping obtained simultaneously with theoptical images is as important as those images and is especially helpful in biological applications, where samples often exhibit complex morphology. Furthermore, the combination of NSOM and ultrafast excitation by apulsed laser has advantages over STM andAFM because the temporal resolution provided
by this combination is in the femtosecond regime. NSOM has thus become a valuable tool
applicable for a wide range of studies in physics, chemistry, and biology.

CCD

Controller

ScannerFig. 1 Schematic illustration of (a) aperture NSOM and (b) apertureless (scattering) NSOM. (c) Schematic of standard
NSOM setup with a local illumination and local collection configuration.

When a small object is illuminated, its finestructures with high spatial frequency generate a localized field that decays exponentiallynormal to the object. This evanescent fieldon the tiny substructure can be used as a local source of light illuminating and scanninga sample surface so close that the light interacts with the sample without diffracting. There
are two methods by which a localized opticalfield suitable for NSOM can be generated. As
illustrated in Fig. 1(a), one method uses a small* Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
Special Research Laboratory for Optical Science, Kanagawa Academy of Science and Technology, Kawasaki, Kanagawa 213-0012, Japan**Spectroscopic Products Division, JASCO Corporation, Hachioji, Tokyo 192-8537, Japan

22

JSAPI̲Vol5-new.p65

JSAP International No.5 (January 2002)

Page 2202.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

aperture at the apex of a tapered optical fiber
coated with metal. Light sent down the fiber
probe and through the aperture illuminates asmall area on the sample surface. The fundamental spatial resolution of aperture NSOM isdetermined by the diameter of the aperture,
which ranges from 10 to 100 nm.
In the other method, called apertureless(or scattering) NSOM and illustrated in Fig.
1(b), a strongly confined optical field is created by external illumination at the apex of
sharpened metal or dielectric tip. Spatial resolution approaching the atomic scale is expected, and laboratory experiments haveyielded resolutions ranging from 1 to 20 nm.
A rather large (diffraction limited) laser spot
focused on a tip apex frequently causes anintense background that reduces the signalto-noise ratio. This contrasts with what is done
in aperture NSOM, where the aperture serves
as a localized light source without any background. The general applicability of theapertureless method to a wide range ofsamples is currently being investigated.
The simplest setup for aperture NSOM, aconfiguration with local illumination and local collection of light through an aperture, isillustrated in Fig. 1(c). The probe quality andthe regulation system for tip-sample feedback
are critical to NSOM performance, and mostNSOMs use a method similar to that used in
an AFM, called shear force feedback, the regulation range of which is 0-20 nm. The lightemitted by the aperture interacts with thesample locally. It can be absorbed, scattered,or phase-shifted, or it can excite fluorescence.
Which of these occur(s) depends on thesample and produce(s) contrast in the opticalimages. In any case, light emerging from theinteraction volume must be collected as efficiently as possible. When the sample is prepared on a transparent substrate, signal lightis frequently collected with an objective lensarranged in a transmission configuration.

taper structure with a sharp apex, (b) coatingwith a metal (Al, Au, Ag) to obtain an entirely
opaque film on the probe, and (c) the formation of a small aperture at the apex.
There are two methods used to make tapered optical fibers with a sharp tip and reasonable cone angle. One is the heating-andpulling method, where the fiber is locallyheated using a CO2 laser and is subsequently
pulled apart. The other method, based on
chemical etching in a hydrofluoric acid (HF)
solution, is more reproducible production andcan be used to make many probes at the sametime.3,4) A specific advantage of the chemicaletching method is that the taper angle, which
determines the efficiency of light transmission
and will be discussed in the following section,
can be adjusted by changing the compositionof a buffered HF solution. Another important
advantage is the excellent stability of the polarization state of the probe. When the heating-and-pulling method is used, in contrast,
temporal fluctuation of polarization occurs
due to relaxation of strain induced by the production process.
The technique most often used to form asmall aperture is based on the geometricalshadowing method used in the evaporation

3. Fabrication of aperture
probe
Great efforts have been devoted to thefabrication of the aperture probe, which is at
the heart of NSOM. Since the quality of theprobe determines the spatial resolution andsensitivity of the measurements, tip fabrication remains of major interest in near-field
optics. The fabrication of fiber-based opticalprobes can be divided into the three main
steps illustrated in Fig 2: (a) the creation of aTapering
Buffered HF

CO2 Laser

Metallization&
AperturecreationTransmission Coefficient

100

(a)

90º
0.7µm

10-1
60º

10-2
10-3
10-4

Metal Evaporation

10

FIB

Fig. 3 (a) Plots of transmission coefficient of aperture probe as a function ofaperture diameter for single-tapered and double-tapered probes with coneangles of 60° and 90°. A He-Ne laser with a wavelength of 632.8 nm was used
for this measurement. (b) Comparison of collection efficiency of an objective
and a probe with a 70-nm aperture. Photoluminescence intensities collectedfrom a single quantum dot are plotted as a function of distance between theaperture and the surface.

PL Intensity [arb. units](b)Fig. 2 Fabrication of an aperture NSOM probe: creation of taper structure, metallization of exterior
surface, and formation of aperture at the apex. The inset shows scanning electron micrographs of 50nm and 70-nm apertures made by the impact method.

20º

0.7µm

100
Aperture Diameter [nm]1000Collection by probe

8

aperture diameter=70 nm

6Quantum dot

Lens

4

Objective (NA 0.8)

Collection by lens

2
0

Probe
0

50

100

150

200

Aperture-Surface Distance d [nm]JSAP International No.5 (January 2002)

JSAPI̲Vol5-new.p65

Page 23d

02.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

23

(a)

(b)

(c)

Intensity(d)Probe Position

Fig. 4 (a) Typical near-field fluorescence image of single dye molecules. (b) Magnified view of one of the bright spots inpart (a). Irreversible photobleaching occurred at the moment indicated by the arrow. (c) One of the highest-resolution
images obtained by using a 20-nm aperture whose diameter was determined as shown by the scanning electron micrographin part (d).

of metal. Since the metal is evaporated at anangle from behind, the coating rate at theapex is much smaller than that on the sides.
The evaporated metal film generally has agrainy texture, resulting in an irregularly
shaped aperture with asymmetric polarization
behavior. The grains also increase the distance
between the aperture and the sample, not only
degrading resolution but also reducing the intensity of the local excitation. A method formaking a high-definition aperture probe by
milling and polishing using a focused ion beam
has been developed,5) as has a simple method
based on the mechanical impact of the tip on
a suitable surface.6) In both methods, the resulting probe has a flat end and a well-defined circular aperture. Furthermore, the impact method assures that the aperture planeis strictly parallel to the sample surface, which
is important in minimizing the distance between the aperture and the sample surface.

4. Transmission efficiency of
the aperture probe
To achieve the ultimate performance inaperture NSOM, the throughput of light aswell as the quality of the aperture should be

24

JSAPI̲Vol5-new.p65

optimized. In addition to contributing to thehigh sensitivity of NSOM measurements, optical probes with high transmission efficiency
will open up attractive new areas of research,such as nonlinear processes and optical manufacturing on the nanometer scale. It is therefore of great importance to understand thelimitations and possibilities of aperture probeswith respect to light transmission.
The tapered region of the aperture probe
is considered to be a metal-clad opticalwaveguide. The mode structure in a metallic
waveguide is completely different from thatin an unperturbed fiber and is characterized
by the cutoff diameter and absorption coefficient of the cladding metal. Theoretical andsystematic experimental studies have confirmed that the transmission efficiency of the
propagating mode decreases in the region
where the core diameter is smaller than half
the wavelength of the light in the core. The
power that is actually delivered to the aperture depends on the distance between theaperture plane and the plane in which theprobe diameter is equal to the cutoff diameter, which distance is determined by the coneangle of the taper. We therefore proposed adouble-tapered structure with a large coneangle (insets in Fig. 3 (a)).7) This structure iseasily realized using a multi-step HF etching
technique. In Fig. 3(a) the transmission coefficient (the ratio of the light power emitted by
the aperture to the power coupled into thefiber) is plotted against aperture diameter for
conventional single-tapered probes and for
double-tapered probes with different coneangles. It is clear that the transmission efficiency of the double-tapered structure with alarge cone angle is two orders of magnitude
greater than that of the single-tapered probewith a small cone angle. More sophisticatedstructures, such as a triple-tapered probe, havealso been proposed for specific applications.8,9)Superior collection of the locally emitted
signal is important for the observation of
opaque materials like semiconductors. The
collection efficiency of the 70-nm apertureprobe is compared with that of an objectivein Fig. 3(b), where the photoluminescence intensity collected from a single quantum dot isplotted as a function of distance between theaperture and the surface. Within 15 nm of thesurface, the amount of light collected by theaperture probe increases rapidly and reaches
a value as great as the amount collected byan objective with a numerical aperture of 0.8.
This suggests that fabrication of the flat-ended
probe makes a considerable contribution to
the efficient interaction of the aperture andevanescent field in the vicinity of the sample.
The overall throughput (or collection efficiency) of light is determined by various factors such as the wavelength of propagating
light and the corresponding dielectric constantof the cladding metal, as well as by the structure of unperturbed fiber. Numerical analysis
and computational simulation of light propagation is a useful and efficient way to obtain
the information needed to make an optimized
structure.10,11)

5. Fluorescence imaging spectroscopy
NSOM fluorescence measurements are thesimplest and most informative method because
they provide high-contrast imaging of
nanostructures, revealing the chemical composition and molecular structure as well as thedefects and dopants in semiconductors. Highresolution optical imaging of biological samples
with fluorescence labels is also a promising fieldof application. NSOM has the potential to image the distribution of such labels down to the

JSAP International No.5 (January 2002)

Page 2402.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

no energy is transferred when the molecule isbelow the aperture hollow. Because the interaction distance in the process of energy transfer is only a few nanometers, we expect toreach a spatial resolution less than 10 nm.
W i t h t h e r e c e n t p ro g re s s i n t h enanostructuring of semiconductor materials
and in the applications of these nanostructured
materials in optoelectronics, NSOM microscopy
and spectroscopy have become important tools
for determining the local optical properties ofthese structures. In single-quantum-dot spectroscopy, NSOM provides access to individual
quantum dots (QDs), an ensemble of which
exhibits inhomogeneous broadening due to thewide intra-ensemble distribution of sizes andstrains.13) NSOM can thus elucidate the natureof QDs, including the narrow optical transition
arising from the atom-like discrete density ofstates. Figure 5(b) is a room-temperature photoluminescence (PL) image of individual selfassembled InGaAs QDs grown on a GaAs substrate, the structure of which is illustrated inFig. 5(a). A typical PL spectrum of a single QD
is shown in Fig. 5(c). At an appropriate excitation density, emission lines from the first excited state (Eex) as well as the ground state (Eg)can be seen. Here we focus on the broadeningof homogeneous linewidth, which is deter-(a)mined by the dephasing time of the electron
system. Single-QD PL spectra of several tens of
QDs were measured and the homogeneous
linewidths of ground-state emission are plotted as a function of interlevel spacing energy(∆E=Eex-Eg) in Fig. 5(d).14) It can be seen thatthe homogeneous linewidth increases with ∆E,
which reflects the size of QDs (quantum confinement should cause ∆E to increase when the
QD size is deceased). Such a dependence oflinewidth on QD size can be reproduced bytheoretical calculation, and it gives us information important for optimizing the QD size fordevice applications.
In contrast to the well-defined quantum
confined systems such as QDs grown in selfassembled mode, the more common disordered systems with local potential fluctuations
still leave open questions. To fully understandsuch complicated systems from information
gained by optical spectroscopic study, we willneed a spatial resolution much better than the
100 nm required for conventional NSOM measurement of semiconductor materials. We
therefore used an NSOM probe with high collection efficiency for 10-nm imaging spectroscopy of a semiconductor alloy with composition fluctuation. The PL from a single GaNAs
quantum well was investigated using NSOMInGaAs Quantum dot (QD)(b)

GaAs/AIGaAs
Cap layers

GaAs Substrate
InGaAs Wetting layer

(c)

20

300 K

2

Linewidth [meV]3
PL Intensity [arb. unit]

level of single fluorophores. Furthermore, thepresence of the probe (metal aperture) alters
the fluorescence lifetime and emission pattern,
making it possible to control radiation in theimmediate vicinity of the probe (i.e., locallywithin the nanoenvironment).
Although NSOM offers attractive possibilities with regard to the detection of single
molecules, the current spatial resolution of 100
nm is not sufficient for application to actual
biological samples. Imaging with a very small
aperture (<30 nm) has often been impossible
because the transmission efficiency decreases
drastically with aperture diameter. Moreover,even a small grain on the aperture plane isproblematic because the consequent increaseof the aperture-sample distance reduces thespatial resolution. As mentioned in the previous section, however, tailored probes with ahigh-quality aperture have solved this problem.
We have made fluorescence images ofsingle dye molecules by using an aperture significantly smaller than usual. 12) Cy 5.5 molecules were dispersed on a quartz substrate
and illuminated by He-Ne laser light throughan aperture made of gold. The fluorescencefrom a single molecule was collected by thesame aperture or by an objective in transmission configuration. Figure 4(a) shows a typical fluorescence image of individual molecules,
and Fig. 4(b) is a magnified view of one of thebright spots in Fig. 4(a). The diameter of eachspot was estimated to be 30 nm, which corresponds to the aperture diameter produced
by using the impact method. Step-like digital
bleaching, which occurred at the moment indicated by the arrow, confirms that we actually observed single molecules. The highest
resolution we achieved is shown in Fig. 4(c).
The diameter of the fluorescence spot was as
small as 15 nm, which is less than the aperture diameter evaluated by using a scanningelectron microscope as shown in Fig. 4(d). This
result is inconsistent with conventional NSOM
observations; the optical spot generated at theaperture should be larger than the physicalaperture diameter because of the finite penetration of light into the coated metal. One
possible explanation for this discrepancy isbased on the contribution of energy transferfrom an excited molecule to the metal aperture: When the molecule is beneath the metal
coating, the energy transfer shortens the fluorescence decay time and thus reduces emission intensity dramatically. On the other hand,

Eg EexEg Eex1∆Eg-ex

(d)

10

large

0

1.1

1.2
1.3
Photon Energy [eV]

1.4

300 K

0

QD Size

small

20
40
60
Interlevel Spacing ∆Eg-ex [meV]Fig. 5 (a) Sample structure of InGaAs quantum dots (QDs). (b) Room-temperature photoluminescence near-field image ofindividual QDs. (c) Photoluminescence spectrum of a single QD at room temperature. (d) Dependence of the homogeneous
linewidth of ground-state emission on interlevel spacing, which is closely related to the size of QDs.

JSAP International No.5 (January 2002)

JSAPI̲Vol5-new.p65

Page 2502.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

25

at cryogenic temperature. Figure 6(a), comparing far-field (macroscopic) and near-field(microscopic) PL spectra measured at 8 K,
shows that the broad far-field PL spectrum
contains extremely sharp PL lines as well as
broad lines. Figures 6(b) and 6(c), monochromatic PL images obtained in two different areas with the PL detection energy fixed at 1.361
eV, show that spatial features of the potential
fluctuation are clearly visualized when the
resolution is 10 nm. The position dependenceof PL spectra in Fig. 6(d) clarifies that such smallfeatures correspond to the broader emission
while the intensity of narrow emission remains
almost constant in the observation area. Localization and delocalization of excitons should
be caused by the interface roughness of thequantum well and the compositional variationof nitrogen. Systematic studies will provide
insight into the transport and emission properties of complicated electron systems, and it

is noteworthy that 10-nm resolution is sufficient for mapping out the exciton wave functions of quantum confined systems.

6. Ultrafast coherent spectroscopy
Time-resolved optical spectroscopy provides a wealth of information on dynamic processes like the phase and energy relaxation of
excitons in semiconductor nanostrucures.
Real-space diffusion, trapping, and relaxation
processes of photogenerated carriers in lowdimensional semiconductors are of interest
from the viewpoint of fundamental physics aswell as with regard to potential device applications. The combination of femtosecond
spectroscopy with NSOM offers new perspectives for the direct investigation of carrier dynamics on the nanometer length scale.
Coherent control of electronic excitationof the quantum confined system is of great

PL Intensity [arb. units]

(a)
PL

(a)

1.310

1.32

1.34

1.36

1.38

1.40

Photon Energy [eV]

(b)

E1

E0

1.315
1.350
Photon Energy [eV]

(b)

E2

PLE

τ

1.30

importance because of its possible applicationin quantum information manipulations suchas quantum computation. Toda et al. performed near-field coherent excitation spectroscopy of a single QD and generated a coherent superposition of carrier wave functions
with different energies.15) They observed an
InGaAs self-assembled QD at cryogenic temperature. A PL spectrum of the lowest emission (E 0) and a corresponding PL excitation
spectrum with respect to the PL peak areshown in Fig. 7(a). Several sharp lines in the
excitation spectrum can be identified as resonant Raman scattering by localized phonons.
A femtosecond-pulsed laser was tuned to anenergy at which two peaks (peaks E1 and E2 in
Fig. 7(a)) could be excited simultaneously. As
shown in Fig. 7(b), sequence pulses with timedelay τ excited the QD through the apertureprobe, and the PL signal was collected by thesame probe. In Fig. 7(c) the PL intensity de-

1.360

1.370

E2
E1
E0

(d)

(c)

B
C

(c)

D
E
F
G
H

1.34 1.35 1.36 1.37 1.38 1.39

Photon Energy [eV]Fig. 6 (a) Photoluminescence (PL) spectra of single GaNAs quantum well with macroscopic
far-field detection (upper) and near-field detection (lower). (b) and (c) Near-field PL images
obtained in different scanning areas with the detection energy fixed at 1.361 eV (indicatedby the arrow in (a)). (d) Position dependence of PL spectra obtained at every 4-nm stepfrom A to H in (b).

26

JSAPI̲Vol5-new.p65

PL Intensity [arb. units]

PL Intensity [arb. units]

A

0

4

8

12

Time Delay τ [ps]Fig. 7 (a) Near-field photoluminescence (PL) spectrum of a single quantum dot andcorresponding PL excitation spectrum. (b) Schematic illustration of two-pulse coherent
spectroscopy and an energy diagram of the optical transition involved in this experiment.
(c) Quantum beat signal between E1 and E2 resonances.

JSAP International No.5 (January 2002)

Page 26

16

02.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

tected is plotted against pulse delay τ. The
amplitude of the PL intensities exhibits a slow
oscillation superimposed on the exponential
decay. The modulation frequency is consistent
with the energy separation between the two
peaks. Such a beat behavior implies that two
closely spaced quantum states are coherently
coupled with the ground state has been generated. The estimated decay time (dashedline), which is associated with the lifetime of
the localized phonon, is in good agreementwith the inverse linewidth of the two resonances.

7. Polarization microscopy
Polarization-contrast NSOM is promisingfor investigating a variety of material characteristics, such as optical anisotropy, magnetooptical effects and electron spin dynamics.
Local polarization control and detection in thenanometer region will be directed toward thedevelopment of nano-optoelectronic devices
and nano-recording media. One advantage ofthe tapered fiber probe in polarization NSOM
is that the polarization at the aperture, as
measured in the far field, can be set to any
desired state, with an extremely high extinction ratio. Most conventional polarization
spectroscopy can thus be combined with
NSOM measurement. Here we describe twonovel applications of polarization NSOM: observing the electric field response of liquid crystal (LC) molecules, 16) and imaging phasechange recording marks by using depolarization NSOM.17)
The physical properties of LC display devices are determined by the interaction between the LC and the alignment layer. To investigate the microscopic nature of this interaction, we developed a type of NSOM suitable for the investigation of local orientationof a LC to which an electric field has been
applied. As illustrated in Fig. 8(a), a fiber probe
was brought into a droplet of LC and set inclose proximity to the alignment layer. The LC
orientation at the apex was measured by thechange in the polarization state of transmitted light in a crossed Nicols polarization configuration. The metallized tip was also used
to apply a uniform electric field to LC molecules in the observation volume.
Figures 8(b) and 8(c) respectively show
NSOM images of LC orientation near the damaged alignment layer without and with theapplied electric field. The tip-surface distance
was maintained at 300 nm by utilizing an op-

tical feedback withevanescent illumina(a)tion from the backside of the alignment layer. In Fig.
Analyzer8(b) strong opticalcontrast is seen inLCdropletthe damaged area.
One possible explanation for this contrast is as follows.
(b)(c)
The dark area,
where the alignment
layer was poundedby the probe, wasseriously damaged
and the anchoring
effect was reduced
there. The intensityof the light transmitted through this area
decreased because
the LC molecules inthe damaged area
did not align homoFig. 8 (a) Experimental configuration for liquid crystal (LC) observation. Near-field imaging
g e n e o u s l y. T h e
of LC orientation on a damaged alignment film (b) without and (c) with an applied voltage.
alignment layer inthe pounded area
was squeezed out and into the surroundingat the aperture could be controlled by using
region. The bright circular contrast should corλ/2 and λ/4 waveplates. The internally rerespond to this region where the pre-tilt angle
flected background light with ellipsoidal pois decreased. In Fig. 8(c), on the other hand,larization was rejected using a λ/4 waveplate
there is no contrast due to surface damage
and an analyzer, and the orthogonal compobecause the electric field forces the LC molnent was detected as a signal. Appropriate
ecules to align vertically.
preparation of the fiber probe enables us toReflection-mode NSOM, in which the apachieve high extinction of background with aerture probe both illuminates the sample andratio in excess of 10-5, as plotted in Fig. 9(b).
collects the reflected light, is expected to exIn this measurement we used an ultrasmall
tend the application of NSOM to the indusaperture with a diameter of 15 nm, an SEM
trial field for use in optical data storage andpicture of which is shown as an inset in Fig.
in optical diagnostics (for opaque materials).
9(a). The sample was a phase-change DVDThe difficulty of reflection NSOM is due to its
RAM medium, where signals are recorded bypoor optical contrast, which is in turn due toreversibly changing small regions between thethe strong background signal reflected backamorphous state and crystal state.
from the tapered region of the probe. One
Figures 9(c) and 9(d) are polarization
promising solution is to also use the concept
NSOM images of amorphous recording marksof depolarization NSOM, in which the depowritten by a blue laser diode. The image inlarized (orthogonally polarized) light due toFig. 9(c) was obtained when the λ/4 waveplate
the near-field interaction between the aperand the analyzer were adjusted to minimize
ture and the sample can be detected as a sigthe background signal reflected from the crysnal because the background has been suptal area. Amorphous recording marks are clear,pressed effectively. We used polarizationand the contrast between the amorphous area
NSOM for high-contrast imaging of DVD-RAM
and the crystal area was 8:1, four times therecording marks.
2:1 contrast obtained in the far-field reflecThe experimental setup is shown schetion measurement. To check the polarization
matically in Fig. 9(a). The polarization of lightstate of light reflected from amorphous record-

JSAP International No.5 (January 2002)

JSAPI̲Vol5-new.p65

Page 2702.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

27

(a)
λ/4

θ
λ/4

Raman Intensity [arb. units]

8

Detector

Analyzer(c)(a)

Raman spectrum of PDA is compared with theRaman spectrum obtained when the probe
was 100 nm from the sample. The result assures us that the aperture probe collects the
local Raman signal without any far-field background. Figure 10(b) displays a mapping ofRaman intensity at the C=C peak (1460 cm-1).
The corresponding topographic image does
not exhibit any significant structure or correlation with the Raman image. The properties
imaged depend on the polarization geometry,
and this dependence is useful when we investigate conformation characteristics such as
crystal orientation and the degree of crystal-

Raman
Intensity
1
1

2[ µ m]

FF
01000[ µ m]

1800

Raman Shift [cm-1]

0

Fig. 10 (a) Raman spectra of polydiacetylene thin film obtained in near-field configuration (upper) and that obtained bypositioning the probe 100 nm above the substrate. (b) Near-field mapping of Raman intensity at the C = C peak (1460
cm-1).

28

JSAPI̲Vol5-new.p65

-0.5

0
0.5
θ [degree]

min: 0.4 V max: 5.0 V

Fig. 9 (a) Schematic diagram of setup for depolarization NSOM in the reflection configuration. The inset is a scanningelectron micrograph of the aperture. (b) Evaluation of extinction ratio for rejection of background reflected light. (c) and(d) Near-field images of recording marks of phase-change medium with polarization contrast. The analyzer was adjusted
to minimize the background signal either on (c) the crystal area or (d) the amorphous area.

(b)

NF

1

(d)

min: 0.9 V max: 7.0 V

6

4

2

0

8. Raman spectroscopy
In material science and chemistry, Raman
spectroscopy has been widely used to study
the distributions of phase and stress as wellas to identify molecules and bonds. Raman
microscopy is a valuable tool in silicon integrated circuit technology because the Raman
shifts are indicative of local stress, which candegrade device performance. As the dimensions of device structures move into the submicron regime, we need Raman spectroscopic
measurements with resolution higher thanthat of conventional techniques. One technical challenge of NSOM Raman spectroscopy
is the detection of very low signal levels resulting from the small Raman scattering crosssection. This challenge, however, is expected
to be met by using a high-transmission probethat delivers several tens of microwatts to a
spot on the sample surface 100 nm in diameter and collects the Raman signal with a sensitivity as high as that of an objective with alarge numerical aperture.
As a demonstration of the feasibility of
NSOM Raman spectroscopy, we show the twodimensional mapping of the Raman signal ofa thin film of polydiacetylene (PDA).18) The
same aperture was used for local illuminationof the sample and local collection of theRaman signal. In Fig. 10(a) the near-field(b)

He-Ne Laser

λ/2Intensity [a.u.]

ing marks, we readjusted the analyzer to minimize the signal in the amorphous area. The
extinction ratio reached the same value as that
estimated for the crystal area. The image obtained is shown Figure 9(d), where the contrast is as high as in Fig. 9(c). Although themechanism of contrast generation is not fully
understood, such high-contrast imaging ofrecording marks is intriguing from the viewpoint of recording device applications.

linity of a sample.
Apertureless NSOM with a metallized
sharp tip has attracted growing interest among
those who want to push the limit of spatial
resolution down to 10-30 nm. Since the electric field and the resultant Raman efficiency isdrastically enhanced at the tip, better rejection of stray light as well as high resolution
can be achieved. Hayazawa et al. made Raman
images of dye molecules at a 30-nm spatial
resolution with a reasonable signal intensity.19)
Their experimental configuration is illustrated
in Fig. 11(a). The probe was prepared by metallizing a silicon cantilever with silver, and theapex diameter was approximately 40 nm.
Rhodamine 6G molecules cast on a silver island film were used as the sample, and theunwanted background was suppressed byusing focused evanescent illumination. Ramanpeaks of the C-C vibration modes, obtained
at steps of 30-nm in the line-scanning, areshown in Fig. 11(b). The appearance of new
peaks as well as remarkable changes in peak
position and intensity can be seen clearly insome spectra. These features seem to beclosely related to both a pure electromagnetic
effect and an enhancement due to chargetransfer or bond formation.

JSAP International No.5 (January 2002)

Page 2802.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

Ag-coated cantilever

Raman Intensity [arb. units](a)Rhodamine 6G
Ag film

Illumination

Raman signal

9. Perspectives
The quality and reproducibility of NSOM
measurements have been greatly improved in
the last few years by the introduction of highdefinition fiber probes made by combining
chemical etching with ion-beam milling or byusing a simpler impact method. Probes having apertures with diameters as small as 10
nm have been made, and their excellent performance have been demonstrated throughhigh-resolution imaging of semiconductor
nanostuructures and single molecules. The
probes, however, should be improved withrespect to optical transmission, tip stability, anddamage threshold. Microfabricated probes
offer a promising solution to these difficulties,
and are likely to improve experimental reproducibility by providing better defined conditions. Apertureless (or scattering) techniques
will surely improve spatial resolution.
The combination of NSOM with a pulsed
laser not only makes possible ultrafast timeresolved spectroscopy but also enables us toutilize nonlinear optical processes such as twophoton absorption and second harmonic generation. Such multi-photon techniques canhelp improve spatial resolution and avoid problems due to stray light. In some cases, NSOM
imaging based on the nonlinear optical response gives us information on microscopic
structures that is complementary to the information given us by linear optical results. In
observation of the domain structure in magnetic thin films, for example, the linear magneto-optics is sensitive to magnetization components perpendicular to the film plane,
whereas second-harmonic generation is sensitive to the in-plane magnetization.

Fig. 11 (a) Schematic drawing of setup for near-field Raman
spectroscopy with local field enhancement. (b) Near-field
Raman spectra of Rhodamine 6G obtained at every 30-nm stepin line scanning.

(b)

500

1000 1500

2000

Raman Shift [cm-1]

Another way to make NSOM more usefulis to extend the wavelength region in which it
can be used. Ultraviolet light (UV) is very useful in biological studies because certain
biomolecules, like DNA and protein molecules,
can be observed with direct excitation. UV
nano-lithography will also be an important
application of NSOM. NSOM infrared spectroscopy with nanometer resolution will be required in a variety of areas in material, molecular, and biological science.
In this article we focused on the spectroscopic applications of NSOM and covered onlya small portion of the research currently being pursued in the area of near-field optics.
Insight into the nanometer-scale interaction
between the mesoscopic electronic system andphotons will provide us with the key concepts
underlying the operation of nano-optoelectronic devices. For example, the near-field
optical coupling of nano-electronic systems
can enhance far-field forbidden transitions.
Such alternation of selection rule of opticaltransition will lead to spatiotemporal coherent control of electronic excitation, which canbe utilized as the principal function of
nanometric devices.20) Remarkable progress isalso being made in industrial applications of
NSOM in areas such as optical data storage21)
and optical manufacturing.22)

Acknowledgements
We are grateful to M. Ohtsu, K. Nishi, H.
Saito, M. Takahashi, H. Toriumi, K. Ichihara, K.
Yusu, T. Tadokoro, T. Inoue, F. Sato, S.
Mononobe, K. Matsuda, N. Hosaka, and K. Ikeda
for their assistance and fruitful discussions.

References
1 ) M. Ohtsu, ed.: Near-Field Nano/Atom Optics
and Spectroscopy (Springer-Verlag 1998).
2 ) S. Kawata, ed.: Near-Field Optics and Surface
Plasmon Polaritons (Springer-Verlag 2001).
3 ) T. Pangaribuan, K. Yamada, S. Jiang, H. Ohsawa
and M. Ohtsu: Jap. J. Appl. Phys. 31, L1302(1992).
4 ) S. Mononobe and M. Ohtsu: IEEE Photon.
Technol. Lett. 10, 99 (1998).
5 ) M. Muranishi, K. Sato, S. Hosaka, A. Kikukawa,T. Shintani and K. Ito: Jpn. J. Appl. Phys. 36,
L942 (1997).
6 ) T. Saiki and K. Matsuda: Appl. Phys. Lett. 74,
2773 (1999).
7 ) T. Saiki, S. Mononobe, M. Ohtsu, N. Saito and
J. Kusano: Appl. Phys. Lett. 68, 2612 (1996).
8 ) S. Mononobe, T. Saiki, T. Suzuki, S. Koshihara
and M. Ohtsu: Opt. Commun. 146, 45 (1998).
9 ) T. Yatsui, M. Kourogi and M. Ohtsu: Appl. Phys.
Lett. 73, 2090 (1998).
10 ) H. Furukawa and S. Kawata: Opt. Commun.
132, 170 (1996).
11 ) H. Nakamura, T. Sato, H. Kambe, K. Sawada
and T. Saiki: J. Microscopy 202, 50 (2001).
12 ) N. Hosaka and T. Saiki: J. Microscopy 202, 362(2001).
13 ) T. Saiki, K. Nishi and M. Ohtsu: Jpn. J. Appl.
Phys. 37, 1639 (1998).
14 ) K. Matsuda, K. Ikeda, T. Saiki, H. Tsuchiya, H.
Saito and K. Nishi: Phys. Rev. B 63, 121304(2001).
15 ) Y. Toda, T. Sugimoto, M. Nishioka and Y.
Arakawa: Appl. Phys. Lett. 76, 3887 (2000).
16 ) T. Tadokoro, T. Saiki and H. Toriumi: submitted
to Jpn. J. Appl. Phys.
17 ) T. Saiki, K. Yusu, K. Ichihara and T. Tadokoro:
submitted to Appl. Phys. Lett.
18 ) Y. Narita, T. Tadokoro, T. Ikeda, T. Saiki, S.
Mononobe and M. Ohtsu: Appl. Spectroscopy
52, 1141 (1998).
19 ) N. Hayazawa, Y. Inouye, Z. Sekkart and S.
Kawata: Chem. Phys. Lett. 335, 369 (2001).
20 ) H. Hori: Optical and Electronic Process of NanoMatters, edited by M. Ohtsu (Kluwer 2001).
21 ) S. Hosaka, T. Shintani, M. Miyamoto, A.
Hirotsune, M. Terao, M. Yoshida, K. Fujita and
S. Kammer: Jpn. J. Appl. Phys. 35, 443 (1996).
22 ) T. Kawazoe, Y. Yamamoto and M. Ohtsu: Appl.
Phys. Lett. 79, 1184 (2001).

JSAP International No.5 (January 2002)

JSAPI̲Vol5-new.p65

Page 2902.1.29, 1:29 PM

Adobe PageMaker 6.5J/PPC

29